Thursday, December 31, 2009

Earlier this year, I made a post that examined whether certain NHL arenas systematically undercount or overcount shots on goal. My methodology involved comparing each team's seasonal home and road splits from 1995 until 2008 in terms of shots on goal . More particularly, the total number of shots taken by both teams in a given team's road games was compared to the total number of shots taken by both teams in that same team's home games.

Where the home-road split revealed a discrepancy in recorded shots, I then looked at the shooting percentage data to determine whether there was, in fact, a recording bias. I reasoned that if a discrepancy was due to bias, rather than randomness or other factors, there ought to a concomitant increase (in the case of undercounting) or decrease (in the case of overcounting) in the shooting percentage of both teams in games played in the arena in question.

Two of the arenas that I suspected might be overcounting shots were Bank Atlantic Center in Sunrise, the home of the Panthers, as well as Continental Airlines Arena (and, perhaps, the Prudential Center as well) in New Jersey. While the data on shooting percentage suggested that shots were likely undercounted in New Jersey, the same was not true of Florida.

The purpose of this post is to take a somewhat more refined look at the topic so as to properly determine the existence of bias. While my initial post looked at overall shot totals and overall shooting percentage, it failed to consider the influence of specific game states, such as special teams play and the playing to the score effect. As both of these factors are known to influence shots on goal as well as shooting and save percentage, merely examining the data in aggregate can be more misleading than illuminating. In order to mitigate these concerns, the data below has been broken down according to game situation.

Firstly, the data for Florida. Presented below is a table comparing the number of shots taken by both teams in Florida road games versus Florida home games, broken down by game state and season. Shots that resulted in an empty net goal have been excluded. This essentially confirms what was already known -- that more shots are recorded in Sunrise than elsewhere. Interestingly, the putative bias appears to be confined to even strength, with no effect on special teams.

I've also prepared a similar table that compares the shooting percentage (again, for both teams) in Florida road and home games. As with the previous table, the percentages do not include empty net goals.

Looking at the data, it's difficult to argue for any sort of shot recording bias. The aggregate shooting percentage in Florida home games is identical to the aggregate shooting percentage in Florida road games. The same is essentially true at even strength with the score tied. If shots were, in fact, being overcounted, then one would expect to to observe a lower shooting percentage in Florida home games. But such is not the case.

In the comment thread of this post made by the Contrarian Goaltender, Vic Ferrari surmised that some of the apparent shot recording biases are not biases at all, and that some arenas really do consistently feature more or fewer shots than average, perhaps due to team style, strategy or some other like factor. I think that's probably the best explanation in this case. The Panthers, for whatever reason, seem to play a more exciting style of hockey at home, which serves to increase the shots on goal numbers while leaving the shooting percentage data unaffected. This is consistent with the shot discrepancy being restricted to even strength.

The data for New Jersey tells a different story. Unlike in Florida, New Jersey home games have featured a deficit of shots, rather than an excess.

More significantly, however, this deficit in shots has been accompanied by an increase in the shooting percentage in Devil home games. This implies that the deficit is due to recording bias, rather than some other factor.

Looking solely at even strength play, the shooting percentage in Devil road games from 2003-04 to 2008-09 has been nearly an entire percentage point lower than the shooting percentage in Devils home games during the same period. While the difference may not seem large, it is greater than what one would expect to observe through chance alone. I've included a separate table below that shows a range of expected shooting percentage values, expressed in the form of confidence intervals, for both EV play with the score tied as well as for EV play in general.

This table shows the range in values where one would expect the overall shooting percentage for New Jersey home games to be found, during the period under review (2003-04 to 2008-09), if it is assumed that:

There is no shot recording bias

The 'true' shooting percentage in Devils home games is equivalent to that observed in Devils road games.

As a specific example, consider the Devils home-road splits at EV. The Devils and their opponents had a combined EV shooting percentage of 0.072 in Devils road games played between 2003-04 and last season. Thus, it is assumed that the underlying shooting percentage in Devils home games is 0.072. Making the further assumption that shots are recorded accurately in New Jersey, the table shows the range in the 'expected' shooting percentage for Devils home games. So, for example, if the above assumptions are true, one would expect to see the shooting percentage in Devils home games fall between 0.067 and 0.0777 95% of the time, and between 0.0654 and 0.0795 99% of the time. The observed value was 0.081, which lies outside both confidence intervals.

One final comment: some will have noticed that more shots were recorded in New Jersey home games than road games for both 2007-08 and 2008-09. I take this to mean that the shot recording bias is likely no longer in existence. While it is true that the shooting percentage in Devils home games continues to be higher than in Devils road games, the difference is probably meaningless in the absence of an actual difference in recorded shots. Perhaps the switch to a different arena was accompanied by a change in shot recorders.

Sunday, December 20, 2009

As a continuation of a previous post, I figured that I'd throw up EV data for the first 500 games of this season.

I've included both overall EV data, as well as data on even strength play with the score tied. Empty net goals have been excluded.

The first sheet shows data for games 1-500, the second for games 1-252, and the third for games 253-500. I've also included a fourth worksheet that compares each team's corsi ratio with the scored tied from the 1st half of the year (games 1-252) to the 2nd half (253-500).

Some comments:

It pains me to admit it as a Habs fan, but Gabe Desjardins is absolutely correct -- the Canadiens look like a terrible team by the numbers. Over the last 250 some games, the Habs have had the worst corsi ratio in the league at EV with the score tied, and by a large margin at that. The scoring chance numbers don't look any better.

To make matters worse, the Habs have also been getting bombed on special teams. In the same 250 game period, they've given up almost twice as many shots on the PK as they've accumulated on the powerplay (55 vs. 98). In fairness to Montreal, that has more to do with their league worst penalty differential than it does with special teams performance per se. In any event, the numbers aren't good.

Curiously, the team's underlying numbers were actually quite respectable over the first 250 some games. It'll be interesting to see where they end up.

On the other side of things, the Ducks appear to have improved considerably at EV relative to the first 250 games. While their current record may not be impressive, they're definitely trending in the right direction. If they can find a way to take fewer penalties, they should be able to at least compete for a playoff spot.

Phoenix continues to perform well at EV. The season is almost halfway over at this point and the Coyotes currently have the 2nd best corsi ratio in the league when the score is tied. This is surprising considering that they were 28th last year in this regard.

I'm not entirely sure on how to account for their turnaround, although I suspect that it boils down to two things. For one, they've gotten rid of and/or sent to the minors a lot of guys that were really hurting them last year (Turris, Lindstrom, Hale, Fedoruk, Lisin, Porter, and Carcillo). The replacements -- Lombardi, Prucha, Fiddler, Lang, Vandermeer, Aucoin, and Vrbata -- are demonstrably better hockey players.

Secondly, I think that the coaching change has likely had an effect as well. Tippett's teams in Dallas were consistently able to outshoot the other guys, both at EV and overall. While it's hard, if not impossible, to quantify his contribution, I think that it's safe to say that he's an upgrade over a relatively inexperienced Gretzky.